Marker means are currently detected by means of a detection means (e.g., a camera or ultrasound detector). The marker means typically comprise three markers arranged in a fixed and predetermined location relative to each other and can be mechanically connected. The markers can be passive or active markers, wherein passive markers reflect signals (e.g., waves and/or radiation) emitted in their direction, and active markers are themselves the source of the signals (e.g., waves and/or radiation). The signals emitted from the (active or passive) markers, which can be wave signals or radiation signals, are detected by the detection means. In order to establish a position of the marker means relative to the detection means, the marker means is preferably moved to provide the detection means with different views of the marker means. On the basis of this, the location of the marker means relative to the detection means then can be determined in a known way, in particular in a spatial reference system. Reference is made in this respect to DE 196 39 615 A1 and the corresponding U.S. Pat. No. 6,351,659, each of which are hereby incorporated by reference in their entirety.